Tailoring barrier properties of thermoplastic corn starch-based films (TPCS) by means of a multilayer design

Tailoring barrier properties of thermoplastic corn starch-based films (TPCS) by 1 means of a multilayer design 2 3 María José Fabra 1* , Amparo López-Rubio 1 , Luis Cabedo 2 and Jose M. Lagaron 1 4 5 1 Food Safety and Preservation Department, IATA-CSIC, Avda. Agustin Escardino 7, 6 46980 Paterna (Valencia), Spain, email: m[email protected] 7 2 Grupo de Polímeros y Materiales Avanzados (PIMA), Universitat Jaume I, Castellón, 8 España 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Abstract 26 This work compares the effect of adding different biopolyester electrospun coatings 27 made of polycaprolactone (PCL), polylactic acid (PLA) and polyhydroxybutyrate 28 (PHB) on oxygen and water vapour barrier properties of a thermoplastic corn starch 29 (TPCS) film. The morphology of the developed multilayer structures was also examined 30 by Scanning Electron Microscopy (SEM). Results showed a positive linear relationship 31 between the amount of the electrospun coatings deposited onto both sides of the TPCS 32 film and the thickness of the coating. Interestingly, the addition of electrospun 33 biopolyester coatings led to an exponential oxygen and water vapour permeability drop 34 as the amount of the electrospun coating increased. This study demonstrated the 35 versatility of the technology here proposed to tailor the barrier properties of food 36 packaging materials according to the final intended use. 37 38 39 Keywords: TPCS, Electrospinning, Multilayer, Barrier properties, Biopolyesters. 40 41 1. INTRODUCTION 42 The use of biopolymers has received increased attention in the last decades as potential 43 substitutes for conventional polymers in a broad range of applications. Among 44 biopolymers, polysaccharides, like starch, are interesting renewable resources that have 45 different applications. Indeed, the introduction of starch in the plastic sector has been 46 motivated by its low cost and biodegradability and by the fact that it is available in large 47 quantities (Xu et al., 2005). However, starch cannot be processed through conventional 48 plastic equipment without further modification because its degradation begins at a 49 temperature lower than its melting point (Avérous, 2004). By the addition of water or 50 other plasticizers such as glycerol or sorbitol, the native crystalline structure of starch is 51 irreversibly disrupted (the so-called gelatinization phenomenon) and thus, the granular 52 starch is transformed into a thermoplastic starch (TPS) which vary from a soft material 53 (high plasticizer level) to a brittle material (low plasticizer level) depending on the 54 moisture and plasticizer level (Jiménez et al., 2012). 55 The barrier to water vapor and oxygen are two essential properties to consider in starch56 based materials because oxygen and water molecules can deteriorate food properties. 57 Indeed, one of the main problems of starch-based films is their high water sensitivity 58 arising from their hydrophilic character, which leads to strong plasticization (Yan et al., 59 2012). This effect negatively affects some characteristics such as the oxygen barrier 60 properties, which are excellent at low hydration levels and plasticizer content but 61 decrease as water sorption increases (Jiménez et al., 2013, Yan et al., 2012). Therefore, 62 many research works have focused on improving starch performance either by blending 63 it with other moisture resistant biodegradable polymers such as polylactic acid (PLA) 64 and polycaprolactone (PCL) (Ali Akbari Ghavimi et al., 2015, Ayana et al., 2014, Cai et 65 al., 2014, Matzinos et al., 2002, Ortega-Toro et al., 2015) or through the addition of 66 dispersed nanoreinforcing agents to generate nanobiocomposites (Dean et al., 2008, 67 Zeppa et al., 2009). However, from an industrial implementation point of view, it is 68 important to highlight that complex multilayer structures are suggested as an alternative 69 to improve the performance of biopolymers, being the most efficient form to constitute 70 barrier materials (Fabra et al., 2013, 2014). Whilst this multilayer design has been 71 widely used for synthetic materials, it has been scarcely developed for biodegradable 72 food packaging systems due to technological problems associated to the scaling-up 73 process and multilayer assembly. Nowadays, this methodology is being successfully 74 exploited by means of electrohydrodynamic processing, also known as electrospinning, 75 to improve the barrier and functional performance of biodegradable polymers 76 thermodynamically immiscible with the additional advantages of forming electrospun 77 coatings (Fabra et al., 2014) or bioadhesives (Fabra et al., 2015 ab) which show 78 excellent adhesion between layers, avoiding the use of synthetic adhesives. 79 Taking advantage of the methodology already described, this paper reports, for the first 80 time, a comparative study in which the effect of different amounts of electrospun 81 biopolyesters coatings (polylactic acid –PLA-, polycaprolactone –PCLand 82 polyhydroxybutyrate –PHB-) has been analyzed and compared in terms of barrier 83 efficiency. 84 85 2. MATERIALS AND METHODS 86 2.1 Materials 87 Polyhydroxybutyrate (PHB) pellets were supplied by Biomer (Krailling, Germany). 88 PHB was reported to have 0-40 wt% of plasticizers and an unreported amount of non89 toxic nucleating agents to improve melt processing (Hänggi, 2011). The semicrystalline 90 polylactide (PLA) used was a film extrusion grade produced by Natureworks (with a D91 isomer content of approximately 2%). The molecular weight had a number-average 92 molecular weight (Mn) of ca. 130,000 g/mol, and the weight average molecular weight 93 (Mw) was ca. 150,000 g/mol as reported by the manufacturer. The polycaprolactone 94 (PCL) grade FB100 was supplied by Solvay Chemicals (Belgium). 95 Corn starch (CS) was kindly supplied by Roquette (Roquette Laisa España, Benifaio, 96 Spain) and glycerol (Panreac Quimica, S.A. Castellar Del Vallés, Barcelona, Spain) was 97 used as plasticizer. 98 N,N-dimethylformamide (DMF) with 99% purity and trichloromethane (99% purity) 99 were purchased from Panreac Quimica S.A. (Barcelona, Spain). 2,2,2-Trifuoroethanol 100 (TFE) with 99% purity were purchased from Sigma-Aldrich (Spain). All products were 101 used as received without further purification. 102 103 2.2. Preparation of films 104 2.2.1 Preparation of thermoplastic corn starch films (TPCS) 105 Corn starch and glycerol, as plasticizer, were dispersed in water using a polymer: 106 glycerol: water ratio of 1:0.3:0.5 (w/w/w) and the dispersion was melt-mixed in a 107 Brabender Plastograph internal mixer at 130ºC and 60 rpm for 4 minutes. The mixture 108 was then spread evenly on Teflon and placed in a compression mould (Carver 4122, 109 USA) at a pressure of 30000 lbs and 130ºC for 5 minutes. 110 111 2.2.2 Preparation of multilayers TPCS systems 112 TPCS films were coated with PHB, PLA or PCL mats produced by means of the 113 electrospinning technique. PHB solutions in 2,2,2-trifluorethanol having a total solids 114 content of 10 wt.% were used to generate the electrospun fibres. The PLA and PCL 115 electrospinning solutions were prepared by dissolving the required amount of the 116 biopolymer, under magnetic stirring, in a solvent prepared with a mixture of 117 trichloromethane (TCM):N,N-dimethylformamide (DMF) in order to reach a 5 or 12 % 118 in weight (wt.-%) of PLA and PCL, respectively. The TCM:DMF ratio used for PLA 119 and PCL was 85:15 and 65:35, respectively. 120 PHB, PLA or PCL fibre mats were directly electrospun onto both sides of the TPCS 121 films by means of a Fluidnatek® electrospinning pilot plant equipment from Bioinicia 122 S.L. (Valencia, Spain) equipped with a variable high-voltage 0-60 kV power supply. 123 Biopolyester solutions were electrospun under a steady flow-rate using a motorized high 124 throughput multinozzle injector, scanning vertically towards a metallic grid used as 125 collector, in which the neat TPCS film was attached. The distance between the needle 126 and the collector was 20, 24 and 31 cm for PHB, PLA and PCL, respectively, and the 127 experiments were carried out at ambient temperature. The voltage of the collector and 128 injector were set at 24 kV and 19 kV, respectively. 129 Different deposition times (0, 2, 10, 20, 40, 60 and 90 minutes), were evaluated in the 130 TPCS film to see how deposition time affected barrier properties. The total amount of 131 electrospun material (mg cm -1 ) was estimated by weighing the TPCS film before and 132 after collection of the electrospun material. 133 With the aim of obtaining transparent and continuous outer layers based on PHB, PLA 134 or PCL, an additional heating step was applied. Coated TPCS films were placed 135 between hot plates at 160ºC to melt and homogenize the PHB or PLA phase and 60ºC to 136 melt the PCL layer. 137 138 2.3. Characterization of films 139 140 2.3.1. Scanning Electron Microscopy (SEM) 141 A Hitachi S-4800 microscope (Hitachi High Technology Corp., Tokyo, Japan) was used 142 to observe the morphology of films cross-sections. Cross-sections of the samples were 143 prepared by cryo-fracture of the films using liquid N 2 . The samples were mounted on 144 bevel sample holders with double-sided adhesive tape, and sputtered with Au/Pd under 145 vacuum. Samples were observed using an accelerating voltage of 10 kV and a working 146 distance of 12–16 mm. Layer thicknesses were measured by means of the Adobe 147 Photoshop CS3 extended software from the SEM micrographs in their original 148 magnification. 149 150 2.3.3. Barrier properties 151 152 2.3.3.1 Water Vapour Permeability (WVP) 153 The WVP of TPCS and multilayer structures was determined by using the ASTM 154 (2011) gravimetric method using Payne permeability cups (Elcometer SPRL, 155 Hermelle/s Argenteau, Belgium) of 3.5 cm diameter. For each type of samples, 156 measurements were done in triplicate and water vapour permeability was carried out at 157 25ºC and 0-100% relative humidity gradient, which was generated by using dry silica 158 gel and distilled water, respectively. The cups were weighed periodically (0.0001 g) 159 after the steady state was reached. Cups with aluminium films were used as control 160 samples to estimate solvent loss through the sealing. Water vapour transmission rate 161 (WVTR) was calculated from the steady-state permeation slopes (8 points) obtained 162 from the regression analysis of weight loss data vs. time (Eq. 1), and weight loss was 163 calculated as the total cell loss minus the loss through the sealing. 164 165 WVTR = ∆ m / ( ∆ t · A) (Eq. 1) 166 where ∆m/∆t, is the weight of moisture loss per unit of time (Kg/s); A, the film area 167 exposed to moisture transfer (m 2 ). 168 Water vapour permeance was calculated using equation 2 as a function of p 1 (water 169 vapor pressure on the film’s inner surface) and p 2 (pressure on the film’s outer surface in 170 the cabinet). 171 Permeance = WVTR / (p 1 - p 2 ) (Eq. 2) 172 Water vapour permeability (WVP) was obtained by multiplying the permeance by the 173 average film thickness as specified in equation 3: 174 WVP = permeance · thickness (Eq. 3) 175 Films thickness was measured in at least 5 different points using a digital micrometer 176 (Mitutoyo, Spain) with ± 0.001 mm accuracy. 177 178 2.3.3.2 Oxygen permeability (O 2 P) 179 The O 2 P was derived from oxygen transmission rate (OTR) measurements recorded, in 180 triplicate, using an Oxygen Permeation Analyzer M8001 (Systech Illinois, UK) at 80% 181 RH and 23ºC. A sample of each multilayer film (5 cm 2 ) was placed in the test cell and 182 pneumatically clamped in place. The samples were previously purged with nitrogen in 183 the humidity equilibrated samples, before exposure to an oxygen flow of 10 mL min -1 . 184 In order to obtain the oxygen permeability (OP) (Eq. 4), film thickness was considered 185 in each case. 186 OP = permeance · thickness (Eq. 4) 187 188 189 2.3.4. Contact Angle Measurements 190 Measurements of contact angle were performed at room conditions (ca. 23ºC and 53% 191 RH) in a Video-Based Contact Angle Meter model OCA 20 (Data Physics Instruments 192 GmbH, Filderstadt, Germany). Data were obtained by analysing the shape of a distilled 193 water drop after it had been placed over the film for 5 s. Image analyses were carried 194 out by SCA20 software. At least, eight replicates were made for each sample. 195 196 2.4. Statistical Analysis 197 Statistical analysis was performed using the analysis of variance procedure (ANOVA) 198 with StatGraphics Plus version 5.1 (Statistical Graphics Corp.). Fisher's Least 199 Significant Difference (LSD) test was applied to detect differences of means, and 200 p<0.05 (95% significant level) was considered to be statistically significant. 201 202 3. RESULTS AND DISCUSSION 203 3.2 Microstructure of multilayer films 204 Since it is well-known that barrier properties of biopolymers are strongly related to their 205 morphology, SEM was used to evaluate the films’ homogeneity, layer structure, 206 presence of pores and cracks, surface smoothness and thickness. SEM micrographs of 207 the surface images of the multilayer TPCS-based films are shown in Figure 1. TPCS 208 film presented homogeneous and smooth surfaces, without visible pores and cracks (see 209 Figure 1). Besides, it was clearly observed that annealing the PCL, PLA and PHB fibres 210 favoured the formation of a continuous coating layer which could contribute to improve 211 the barrier properties of the TPCS films. 212 The cross-section image of the TPCS film showed a compacted structure with absence 213 of intact starch granules, demonstrating the effectiveness of the destructuration and 214 thermo-compression processes (cf. Figure 2A). Representative images of the multilayer 215 Cai, J., Xiong, Z., Zhou, M., Tan, J., Zeng, F., Ma, M., Lin, S. and Xiong, H. (2014). 365 Thermal properties and crystallization behavior of thermoplastic starch/poly(ɛ366 caprolactone) composites. Carbohydrate Polymers, 102, 746-754. DOI: 367 10.1016/j.carbpol.2013.10.095 368 Chan, R.T.H, Marçal, H., Ahmed, T., Russel, R.A., Holden, P.J., and Foster, L.J.R. 369 (2013). Poly(ethylene glycol)-modulated cellular biocompatibility of 370 polyhydroxyalkanoate films. Polymer International, 62, 884-892. DOI: 10.1002/pi.4451 371 Chalco-Sandoval, W., Fabra, M.J., Lopez-Rubio, A., Lagaron, J.M. (2014). Electrospun 372 heat management polymeric materials of interest in food refrigeration and packaging. 373 Journal of Applied Polymer Science, 131 (1) 40661. DOI: 10.1002/app.40661 374 Darie, R.M., Pâslaru, E., Sdrobis, A., Pricope, G.M., Hitruc, G.E., Poiata, A., 375 Baklavaridis, A. and Vasile, C. (2014). Effect of Nanoclay Hydrophilicity on the 376 Poly(lactic acid)/Clay Nanocomposites Properties. Industrial and Engineering 377 Chemistry Research, 7877-7890. DOI: 10.1021/ie500577m 378 Dean, K.M., Do, M. D., Petinakis, E. and Yu, L. (2008): Key interactions in 379 biodegradable thermoplastic starch/poly(vinyl alcohol)/montmorillonite microand 380 nanocomposites. Composite Science and Technology, 68, 1453-1462. 381 DOI:10.1016/j.compscitech.2007.10.037 382 de Campos, A., Tonoli, G.H.D., Marconcini, J.M., MAttoso, L.H.C., Klamczynski, A., 383 Gregorski, K.S., Wood, D., Williams, T., Chiou, B.-S., Imam, S.H. (2013). TPS/PCL 384 Composite Reinforced with Treated Sisal Fibers: Property, Biodegradation and Water385 Absorption. Journal of Polymers and the Environment, 21, 1-7. DOI: 10.1007/s10924386 012-0512-8 387 Fabra, M.J., López-Rubio, A., and Lagaron, J.M. (2013). High barrier 388 polyhydroxyalcanoate food packaging film by means of nanostructured electrospun 389 interlayers of zein. Food Hydrocolloids, 32, 106-114. 390 DOI:10.1016/j.foodhyd.2012.12.007 391 Fabra, M.J., López-Rubio, A. and Lagaron, J.M. (2014). Nanostructured interlayers of 392 zein to improve the barrier properties of high barrier polyhydroxyalkanoates and other 393 polyesters. Journal of Food Engineering, 127, 1-9. 394 DOI:10.1016/j.jfoodeng.2013.11.022 395 Fabra, M.J., López-Rubio, A. and Lagaron, J.M. (2015a). Effect of the film-processing 396 conditions, relative humidity and aging on wheat gluten films coated with electrospun 397 polyhydryalkanoate. Food Hydrocolloids, 44, 292-299. 398 DOI:10.1016/j.foodhyd.2014.09.032 399 Fabra, M.J., López-Rubio, A. and Lagaron, J.M. (2015b). Three-Layer Films Based on 400 Wheat Gluten and Electrospun PHA. Food and Bioprocess Technology, 8(11), 2330401 2340. DOI 10.1007/s11947-015-1590-0 402 Hänggi, U. J. (2011). Biomer biopolyesters. Germany: Krailling. 403 Hutchings, J. B. (1999). Food and colour appearance (2nd ed.). Gaithersburg, 404 M.D.:Chapman and Hall Food Science Book, Aspen Publication. 405 Jiménez, A., Fabra, M.J., Talens, P. and Chiralt, A. (2012). Edible and Biodegradable 406 Starch Films: A Review. Food and Bioprocess Technology, 5(6), 2058-2076. DOI: 407 10.1007/s11947-012-0835-4 408 Jiménez, A., Fabra, M.J., Talens, P. and Chiralt, A. (2013). Phase transitions in starch 409 based films containing fatty acids. Effect on water sorption and mechanical behaviour. 410 Food Hydrocolloids, 30 (1), 408-418. DOI:10.1016/j.foodhyd.2012.07.007 411 Matzinos P, Tserki V, Kontoyiannis A, Panayiotou C. (2002). Processing and 412 characterization of starch/polycaprolactone products. Polymer Degradation and 413 Stability, 1, 17–24. DOI: 10.1016/S0141-3910(02)00072-1 414 Müller, C. M. O., Laurindo, J. B., and Yamashita. (2011). Effect of nanoclay 415 incorporation method on mechanical and water vapor barrier properties of starch-based 416 films. Industrial Crops and Products, 33, 605–610. DOI:10.1016/j.indcrop.2010.12.021 417 Ortega-Toro, R., Contreras, J., Talens, P., Chiralt., A. (2015). Physical and structural 418 properties and thermal behaviour of starch-poly(eopen-caprolactone) blend films for 419 food packaging . Food Packaging and Shelf Life, 5, 10-20. DOI: 420 10.1016/j.fpsl.2015.04.001 421 Pérez-Masiá, R., López-Rubio, A., Fabra, M.J., and Lagaron, J.M. (2013). 422 Biodegradable Polyester-Based Heat Management Materials of Interest in Refrigeration 423 and Smart Packaging Coatings. Journal of Applied Polymer Science, 30(5), 3251-3262. 424 DOI: 10.1002/app.39555 425 Plackett, D., and Siro, I. (2011). Polyhydroxyalkanoates (PHAs) for food packaging. In 426 J. M. Lagaron (Ed.), Multifunctional and nanoreinforced polymers for food packaging. 427 Cambrigde. UK: Woodhead Publishing Limited. 428 Pushpadass, H. A., Marx, D. B., and Hanna, M. A. (2008). Effects of extrusion 429 temperature and plasticizers on the physical and functional properties of starch films. 430 Starch/Stärke, 60, 527–538. DOI: 10.1002/star.200800713 431 Rhim, J. W., Hong, S. I., and Ha, C. S. (2009). Tensile, water vapor barrier and 432 antimicrobial properties of PLA/nanoclay composite films. LWT - Food Science and 433 Technology, 42, 612-617. DOI:10.1016/j.lwt.2008.02.015 434 Sánchez-García, M. D., and Lagaron, J. M. (2010a). On the use of plant cellulose 435 nanowhiskers to enhance the barrier properties of polylactic acid. Cellulose, 17, 987436 1004. DOI: 10.1007/s10570-010-9430-x 437 Sánchez-García, M. D., and Lagaron, J. M. (2010b). Novel Clay-Based 438 Nanobiocomposites of Biopolyesters with Synergistic Barrier to UV Light, Gas, and 439 Vapour. Journal of Applied Polymer Science, 118, 188–199. DOI: 10.1002/app.31986 440 Xu, Y. X., Kimb, K. M., Hanna, M. A., and Nag, D. (2005). Chitosan starch composite 441 film: preparation and characterization. Industrial Crops and Products, 21(2), 185-192. 442 DOI: 10.1016/j.indcrop.2004.03.002 443 Yan, Q., Hou, H., Guo, P., Dong, H., 2012. Effects of extrusion and glycerol content on 444 properties of oxidized and acetylated corn starch-based films. Carbohydrate Polymers, 445 87 (1), 707–712. DOI:10.1016/j.carbpol.2011.08.048 446 Zhijiang, C., Yi, X., Haizheng, Y., Jia, J., Liu, Y. (2016). 447 Poly(hydroxybutyrate)/cellulose acetate blend nanofiber scaffolds: Preparation, 448 characterization and cytocompatibility. Materials Science and Engineering C, 58, 5760, 449 757-767. DOI:10.1016/j.msec.2015.09.048 450 451 452 453 Table 1. Values of a, b coefficients and R 2 in the relationship between WVP 454 and mg (PCL, PLA or PHB) ⋅cm -2 455 Coating layer Model a b R 2 PCL exponential -0.478 4.00E-17 0.992 PLA -0.413 3.00E-17 0.928 without TPCS -0.351 2.00E-17 0.983 PHB -0.425 2.00E-17 0.936 without TPCS -0.378 1.00E-17 0.947 456 Table 2. Values of a, b coefficients and R 2 in the relationship between O 2 P 457 and mg (PCL, PLA or PHB) ⋅cm -2 458 Coating layer Model a b R 2 PCL exponential -0.478 4.00E-17 0.992 PLA -0.413 3.00E-17 0.928 without TPCS -0.351 2.00E-17 0.983 PHB -0.425 2.00E-17 0.936 without TPCS -0.378 1.00E-17 0.947 459 Table 3. Contact angle values of the neat thermoplastic corn starch-based films, the 460 developed multilayer structures and the neat PHB, PLA and PCL films. 461 Coating layer mg ⋅ cm -2 θ (º) 54.8 (3.5) a PCL 0.8 82.2 (2.5) b 3.2 81.9 (3.0) b 7 78.5 (4.4) b PLA 0.5 84.0 (3.8) b 3.2 84.1 (2.6) b 4.9 83.2 (3.6) b PHB 1.5 81.6 (2.6) b 3.2 84.2 (2.4) b 13.6 86.5 (3.1) b PHB 123 (3.5) (*) PLA 86.9 (1.6) (**) PCL 89.5 (1.9) (***) (*) Zhijiang et al., 2016 (**) Darie et al., 2014 (***) Campos et al., 2008 462 a-b: Different superscripts within the same column indicate significant differences among samples (p < 0.05). 463 464 Figure captions 465 Figure 1. Surface images of the neat TPCS film (A) and the developed multilayer films 466 prepared with PCL (B), PLA (C) or PHB (D). 467 Figure 2. Cross-section images of the neat TPCS and multilayer films prepared with 468 PCL at different deposition times (A) TPCS, (B) 20 min, (C) 40 min and (D) 90 min. 469 Figure 3. Cross-section images of the multilayer films prepared with PLA at different 470 deposition times (A) 2 min, (B) 20 min, (C) 40 min and (D) 60 min. 471 Figure 4. Cross-section images of the multilayer films prepared with PHB at different 472 deposition times (A) 20min, (B) 40 min, (C) 60 min and (D) 90 min. 473 Figure 5. Relationships between WVP values vs. the mg electrospun coating ⋅ cm 2 474 (black symbols) and thickness vs. the mg electrospun coating ⋅ cm 2 (white symbols). 475 Figure 6. Relationships between O 2 P values vs. the mg electrospun coating ⋅ cm 2 (black 476 symbols) and thickness vs. the mg electrospun coating ⋅ cm 2 (white symbols). 477 Figure 7. Images of water droplet in contact angle measurements of the uncoated TPCS 478 film (A) and the developed multilayer films prepared with the highest deposited amount 479 of PCL (B), PLA (C) or PHB (D). 480 Figure 1 481 (A) (B) (C) (D) 482 Figure 2 483 (A) (B) (C) (D) PCL coating PCL coating PCL coating 484 485 Figure 3 486 (A) (B) (C) (D) PLA coating PLA coating PLA coating PLA coating 487 488 489 Figure 4 490 (A) (B) (C) (D) PHB coating PHB coating PHB coating PHB coating 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 Figure 5 506 y = 2.7745x R² = 0.9756 0 5 10 15 20 25 0.00E+00 2.00E-13 4.00E-13 6.00E-13 8.00E-13 1.00E-12 1.20E-12 1.40E-12 1.60E-12 1.80E-12 2.00E-12 02468 mg PCL/cm 2 Thickness (µm) WVP (Kg Pa -1 s -1 m -2 ) y = 2.2128x R² = 0.9872 0 5 10 15 20 25 0.00E+00 2.00E-13 4.00E-13 6.00E-13 8.00E-13 1.00E-12 1.20E-12 1.40E-12 1.60E-12 1.80E-12 0246 mg PLA/cm 2 Thickness (µm) WVP (Kg Pa -1 s -1 m -2 ) y = 3.0425x R² = 0.9717 0 5 10 15 20 25 30 35 40 45 50 0.00E+00 2.00E-13 4.00E-13 6.00E-13 8.00E-13 1.00E-12 1.20E-12 1.40E-12 1.60E-12 1.80E-12 0 2 4 6 8 10 12 14 16 mg PHB/cm 2 Thickness (µm) WVP (Kg Pa -1 s -1 m -2 ) 507 508